We are investigating the woodchuck hepatitis virus (WHV) as an animal virus model of hepatitis B virus (HBV). Since both HBV and WHV share many morphological and biochemical properties which are not found in other classes of viruses, we propose that they belong to a novel class of DNA viruses. Both viruses are strongly associated with the occurrence of chronic active hepatitis and hepatocellular carcinoma in chronically infected individuals. The proposed work includes (i) experimental transmission of chronic WHV infections in woodchucks, (ii) studies of the occurrence of hepatocellular carcinomas in experimentally infected woodchucks, (iii) growth of WHV in cultured woodchuck hepatocytes, and (iv) attempts to identify other members of an HBV-like class of viruses.